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April 3, 2012: Republican presidential candidate, former Pennsylvania Sen. Rick Santorum, greets supporters in Cranberry, Pa. Santorum's plan to use state conventions to pull support from Mitt Romney has stalled in North Dakota.AP Photo

Part of the answer lies in the fact that decisions to quit a presidential race have little to do with voters and delegates — and everything to do with what's going on in a candidate's head. Staying in when there's no hope of winning can become a quest for a consolation prize, such as a future Cabinet appointment. But fighting a losing battle also reflects human beings' tendency to gamble, no matter how long the odds. It's also about fighting for one's reputation. Who wants to forever be regarded as a failed presidential candidate?

Santorum clearly doesn't. After Tuesday's losses to Romney in the District, Maryland and Wisconsin, he conjured up comparisons to Ronald Reagan's long-shot GOP nomination bid in 1976:

Reagan "lost almost every early primary. He only won one until May. . . . Everybody told him to get out of the race," Santorum said in a speech in Pennsylvania. "There's one person who understood we don't win by moving to the middle."

Sure, Reagan lost the nomination to Gerald Ford, but Santorum conveniently ignored that, focusing instead on the idea that he might still have a chance.

Behavioral economists Daniel Kahneman and Amos Tversky, pioneers in the field of decision science, discovered that when an individual faces a choice between a sure loss and a gamble, he will go for the riskier option rather than settle for defeat.

For candidates such as Santorum and Gingrich, a loss would be highly public. When you're the exciting new candidate, you're likely to focus on the endless possibilities, and optimism rules the day. But when the polls turn against you, the fear of failure starts to weigh more heavily. Despite whatever they go on to do, the stigma of losing can be nearly impossible to shake. For example, a Los Angeles Times headline last summer read "Failed presidential candidate Al Gore attacks winning candidate Obama over his environment policies." The story had nothing to do with campaigning, yet Gore was still branded a loser.

For politicians like Gingrich — who, as a junior congressman in 1984, told reporters that he had "enormous personal ambition" and wanted to "shift the entire planet" — the idea of being one among many failed candidates might be too much to bear. This can result in doubling down on the unsuccessful course of action. So what if the last primary didn't go as planned? The next one will make up for it.

It's too late for Santorum and Gingrich to avoid loserdom. But if they voluntarily end their campaigns, they'll be actively putting themselves in that camp rather than continuing to run and (oops!) getting stuck there anyway. It's far easier to explain away a failure if you weren't the "decider."

The problem this poses for political candidates is the same one that gamblers face after losing their first $100. The economically rational move would be to cut their losses and walk away. However, dreaming of winning, however slim the odds, makes it difficult for most people to stop playing.

Deciding whether to prolong a candidacy involves estimating the odds of success, which in turn relies on how candidates see themselves and their qualities compared with the rest of the field. Unfortunately, people are not very good at making these judgments. We all tend to overestimate our attributes compared with everyone else's. As Santorum said: "Grandiosity has never been a problem with Newt Gingrich."

But it's not just Gingrich's problem. We all tend to believe that we're attractive, smart individuals. If you put 20 people in a room, it's likely that they would all describe themselves as "above average" on most dimensions — even though this is mathematically impossible.

Even experts are not immune to such errors: Making decisions in their area of expertise, doctors, lawyers and financial managers are just as likely as the general public to fall victim to overconfidence. Political candidates — even smart, knowledgeable ones — are likely to overestimate their chances of winning and, as a campaign goes on, to misjudge their chances of political resurrection. This is why there are so many candidates to begin with.

Of course, not everyone in a campaign is there to win. Some play the game for entirely different reasons. "Statement campaigns" such as Paul's persist longer than expected because bowing out gracefully — and early — doesn't make much of a statement. For some of these conviction politicians, running as the underdog reinforces their self-image. Given that Paul has identified himself as completely out of the mainstream of the Republican Party, it should be no surprise that he is refusing to play by the GOP's rules. After all, what kind of principled stand would it be to leave the race before it's over?

To even get to the point where a presidential campaign is a possibility, politicians must be survivors — smart, lucky and open to risk. In other words, the candidates we get are not drawn at random (though it can sometimes seem like it), but probably possess certain qualities in excess: They are tough, capable and unable to know when it's time to call it quits.

How could they misread reality when there's so much information (i.e. nonstop polling) about how they’re doing? Well, having so much information makes it easy to evaluate a situation in a way that reflects well on ourselves and is consistent with our beliefs. Losing a primary might be interpreted in a candidate's mind as a better-than-expected result (still a defeat, but not as bad as the naysayers predicted!) or even as irrelevant (especially in the states where delegates are allocated proportionally to the share of the vote).

Fortunately, some of these pathologies are avoidable. If candidates set objective benchmarks for themselves before campaigning, it would be harder for them to spin results in their favor.

But candidates are still human — no matter how smart and capable they might be. When Bob Dole said last month that it was "getting close to the point" where Santorum should decide whether to drop out, he acknowledged his own difficulty in giving up his pursuit for the 1988 GOP nomination. "As much as you don't want to do it, sometimes you have to face reality."

And the reality is, we already have a Republican nominee: Mitt Romney.

Jonathan Renshon is a research fellow at the Harvard Kennedy School’s Belfer Center for Science and International Affairs. Jennifer Lerner, a professor of public policy and management at the Kennedy School, directs the Harvard Laboratory for Decision Science. Philip Tetlock is a professor of management and psychology at the University of Pennsylvania.

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